When using heat to increase circulation to an area of the body, moist heat is more beneficial than dry. Moist heat application can be accomplished by a warm bath / shower, hot moist towel or moist heating pad. If you have a dry heating pad, either wrap the pad securely in a plastic baggy, and cover the baggy with a small damp towel or apply a hot moist towel to the skin, followed by a layer of plastic and finally the heating pad on top. Turn the heating pad on and this will produce moist heat.
Heat relaxes the muscles, but can cause or aggravate inflammation. Ice reduces the inflammation, but can cause muscles to tighten; therefore, the process of alternating moist heat and ice is sometimes beneficial.
If increasing circulation to the area in question is the primary goal than hot packs should be used intermittently, and cold applications avoided unless being applied by a trained hydrotherapist.